CD: Deploy to Vercel / Netlify for PR previews

[amithm7]

Vercel seems to have better preview UI / UX for pull requests.

OR Netlify; example preview: github.com/apache/apisix-website/pull/1484#issuecomment-1405984135

https://vercel.com/docs/concepts/deployments/preview-deployments

Then, this whole file for CD to GH pages can be scrapped OR exclude it being run on pull_request.